MEMS devices utilizing a thick metal layer of an interconnect metal film stack

ABSTRACT

A MEMS device, such as an accelerometer or gyroscope, fabricated in interconnect metallization compatible with a CMOS microelectronic device. In embodiments, a proof mass has a first body region utilizing a thick metal layer that is separated from a thin metal layer. The thick metal layer has a film thickness that is significantly greater than that of the thin metal layer for increased mass. The proof mass further includes a first sensing structure comprising the thin metal layer, but lacking the thick metal layer for small feature sizes and increased capacitive coupling to a surrounding fame that includes a second sensing structure comprising the thin metal layer, but also lacking the thick metal layer. In further embodiments, the frame is released and includes regions with the thick metal layer to better match film stress-induced static deflection of the proof mass.

This application is a 371 application of, and claims priority to PCTPatent Application Ser. No. PCT/US2013/056431, filed on 23 Aug. 2013,titled “MEMS DEVICES UTILIZING A THICK METAL LAYER OF AN INTERCONNECTMETAL FILM STACK.”

TECHNICAL FIELD

Embodiments of the invention generally relate to MicroElectroMechanicalSystems (MEMS), and more particularly relate to MEMS accelerometers andgyroscopes utilizing a thick metal layer of an integrated circuit (IC)interconnect metal film stack.

BACKGROUND

MEMS technology is employed in many sensors, such as accelerometers, andgyroscopes. Many MEMS employ a released proof mass that is to experiencea physical displacement relative to a substrate, or frame relativelymore rigidly coupled to the substrate, in response to an externalstimulus. Detection of this physical displacement may be detrimentallyimpacted by insufficient mass and/or residual stresses in the MEMSstructure, particularly stress gradients across a metal film thickness,which cause a structure to statically deflect upon its release from thesubstrate.

To date, commercial MEMS sensor implementations rely on a “two-chip”approach where the MEMS structure is contained on a first chip while acontrol circuit is provided on another (e.g., an ASIC). For thisapproach, the MEMS structure is typically fabricated in bulk siliconsubstrate layers (e.g., an SOI layer), or surface micromachined into apolycrystalline semiconductor layer (e.g., silicon or SiGe). In general,for either of these techniques, the structural semiconductor materialhas very good mechanical properties with low intrinsic stress and alsohas a relative large thickness compared to thin films that are built-upupon the substrate through fabrication of CMOS circuitry, for example.

The two-chip approach however suffers from higher costs and larger formfactors than would a single chip solution. Single-chip approaches havebeen hindered by the need to have the MEMS structures formed fromlow-stress bulk semiconductor films. Thin film structures capable ofachieving high mass while remaining tolerant of intrinsic film stresswould permit greater MEMS performance and permit further integration ofMEMS with conventional integrated circuit technology, such as CMOS,facilitating a single-chip solution.

BRIEF DESCRIPTION OF THE DRAWINGS

The material described herein is illustrated by way of example and notby way of limitation in the accompanying figures. For simplicity andclarity of illustration, elements illustrated in the figures are notnecessarily drawn to scale. For example, the dimensions of some elementsmay be exaggerated relative to other elements for clarity. Further,where considered appropriate, reference labels have been repeated amongthe figures to indicate corresponding or analogous elements. In thefigures:

FIG. 1A is an isometric view of a metal stack including thin metallayers in a region of a MEMS, in accordance with an embodiment;

FIG. 1B is an isometric view of a metal stack including a thick metallayer in certain portions of the MEMS region in FIG. 1A, in accordancewith an embodiment;

FIG. 2 is an isometric view of a MEMS employing the structuresillustrated in FIG. 1B, in accordance with an embodiment;

FIG. 3A is an isometric view of a released MEMS proof mass and releasedframe having approximately equal static deflection from a substrate, inaccordance with an embodiment;

FIG. 3B is a cross-sectional view of a MEMS proof mass employing a thickmetal layer, in accordance with an embodiment;

FIG. 3C is a cross-sectional view of a MEMS frame employing a thickmetal layer, in accordance with an embodiment;

FIG. 4 is a flow diagram illustrating methods of fabricating a MEMS witha thick metal layer, in accordance with an embodiment;

FIG. 5A is cross-sectional view showing a proof mass employing a thickmetal layer in a metal film stack, in accordance with an embodiment;

FIG. 5B is cross-sectional view showing a release of a proof massemploying a thick metal layer, in accordance with an embodiment;

FIG. 6A is a plan view of a MEMS device employing a thick metal layer,in accordance with another embodiment;

FIGS. 6B and 6C are isometric views of the MEMS device depicted in FIG.6A;

FIG. 7 illustrates isometric and expanded views of a mobile computingplatform including a MEMS, in accordance with an embodiment; and

FIG. 8 illustrates a computing device in accordance with oneimplementation of the invention.

DETAILED DESCRIPTION

One or more embodiments are described with reference to the enclosedfigures. While specific configurations and arrangements are depicted anddiscussed in detail, it should be understood that this is done forillustrative purposes only. Persons skilled in the relevant art willrecognize that other configurations and arrangements are possiblewithout departing from the spirit and scope of the description. It willbe apparent to those skilled in the relevant art that techniques and/orarrangements described herein may be employed in a variety of othersystems and applications other than what is described in detail herein.

Reference is made in the following detailed description to theaccompanying drawings, which form a part hereof and illustrate exemplaryembodiments. Further, it is to be understood that other embodiments maybe utilized and structural and/or logical changes may be made withoutdeparting from the scope of claimed subject matter. It should also benoted that directions and references, for example, up, down, top,bottom, and so on, may be used merely to facilitate the description offeatures in the drawings and are not intended to restrict theapplication of claimed subject matter. Therefore, the following detaileddescription is not to be taken in a limiting sense and the scope ofclaimed subject matter is defined solely by the appended claims andtheir equivalents.

In the following description, numerous details are set forth, however,it will be apparent to one skilled in the art, that the presentinvention may be practiced without these specific details. In someinstances, well-known methods and devices are shown in block diagramform, rather than in detail, to avoid obscuring the present invention.Reference throughout this specification to “an embodiment” or “oneembodiment” means that a particular feature, structure, function, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the invention. Thus, the appearances ofthe phrase “in an embodiment” or “in one embodiment” in various placesthroughout this specification are not necessarily referring to the sameembodiment of the invention. Furthermore, the particular features,structures, functions, or characteristics may be combined in anysuitable manner in one or more embodiments. For example, a firstembodiment may be combined with a second embodiment anywhere theparticular features, structures, functions, or characteristicsassociated with the two embodiments are not mutually exclusive.

As used in the description of the invention and the appended claims, thesingular forms “a”, “an” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise. It willalso be understood that the term “and/or” as used herein refers to andencompasses any and all possible combinations of one or more of theassociated listed items.

The terms “coupled” and “connected,” along with their derivatives, maybe used herein to describe functional or structural relationshipsbetween components. It should be understood that these terms are notintended as synonyms for each other. Rather, in particular embodiments,“connected” may be used to indicate that two or more elements are indirect physical, optical, or electrical contact with each other.“Coupled” may be used to indicated that two or more elements are ineither direct or indirect (with other intervening elements between them)physical, optical, or electrical contact with each other, and/or thatthe two or more elements co-operate or interact with each other (e.g.,as in a cause an effect relationship).

The terms “over,” “under,” “between,” and “on” as used herein refer to arelative position of one component or material layer with respect toother components or layers where such physical relationships arenoteworthy. For example in the context of material layers, one layerdisposed over or under another layer may be directly in contact with theother layer or may have one or more intervening layers. Moreover, onelayer disposed between two layers may be directly in contact with thetwo layers or may have one or more intervening layers. In contrast, afirst layer “on” a second layer is in direct contact with that secondlayer. Similar distinctions are to be made in the context of componentassemblies.

As used in throughout this description, and in the claims, a list ofitems joined by the term “at least one of” or “one or more of” can meanany combination of the listed terms. For example, the phrase “at leastone of A, B or C” can mean A; B; C; A and B; A and C; B and C; or A, Band C.

As described in greater detail below, a MEMS device, such as anaccelerometer or gyroscope, is fabricated with interconnectmetallization compatible with a CMOS microelectronic device. Inembodiments, a proof mass has a first body region utilizing a thickmetal layer that is separated from a thin metal layer. The thick metallayer has a film thickness that is significantly greater than that ofthe thin metal layer, advantageously increasing mass. The proof massfurther includes a first sensing structure comprising the thin metallayer, but lacking the thick metal layer, enabling delineation of smallfeature sizes and spaces for an increase in coupling to a second sensingstructure, that also comprises the thin metal layer and lacks the thickmetal layer, formed in an adjacent frame. In further embodiments, boththe proof mass and the flame are released from the substrate and theframe includes regions with the thick metal layer to better match filmstress-induced static deflection of the proof mass. In certain suchembodiments, regions of the frame lacking the thick metal are providedin alignment with thin metal features of the proof mass, and of asimilar lateral dimension. Regions of thin metal frame may, for example,enable film stress in the frame to better match that in the proof mass.In certain such embodiments, mass increases in the frame associated withregions of thick metal are mitigated with larger dimensioned openingspassing through the thin metal layer within the frame relative to thosein the proof mass.

As further described in greater detail below, methods to fabricate MEMSdevices with interconnect metallization thin films may includepatterning a first body region of a proof mass in a metal film stackthat includes a thick metal layer separated from a thin metal layer bydielectric layer. A second body region of a frame may be patterned atleast one of the metal layers and advantageously the thick metal layeras well. At least the proof mass, and advantageously the frame as well,are then released from the substrate.

Generally, embodiments of MEMS structures described herein are suitablefor a wide variety of MEMS devices. In exemplary embodiments, the MEMSdevice is a sensor of rotational velocity (i.e., a gyroscope), or linearacceleration (i.e., an accelerometer). For accelerometer embodiments,one or more of x-axis, y-axis and z-axis acceleration is sensed (withz-axis being perpendicular to a top surface of a substrate over whichthe MEMS is disposed). In embodiments, acceleration is sensed based on ameasurable change in capacitance between two conductive sensingstructures that are electrically insulated/isolated from each other butcapacitively coupled. Other transduction means, such as, but not limitedto inductive coupling structures, may also be utilized. For theexemplary capacitive embodiments described in detail herein, the changein capacitance is achieved through a change in capacitive plate area asa proof mass is displaced (either linearly or rotationally) relative toa frame in response to acceleration, rather than through a change in adistance between the capacitive members. The structures and techniquesfor imparting high mass with interconnect metal films while maintaininggood tolerance to intrinsic film stress described herein may nonethelessbe adapted to other MEMS structures that rely on a change in capacitorplate spacing.

In embodiments, a MEMS device includes thick and thin metal layerswithin a released member. Interconnect metallization employed inmicroelectronics fabrication generally comprises a plurality of metalthin films separated from each other by intervening interlayerdielectrics (ILD). Each metal layer of the metal film stack issignificantly less than 1 μm, and typically no more than 0.5 μm.Advanced CMOS processes may have an interconnect metal stack thatincludes 4-9 levels of such metal layers (or more) with each layer beingapproximately the same thickness, or only slightly thicker (e.g., lessthan twice) as an underlying metal layer. In the exemplary embodiment,the interconnect metallization additionally includes a thick metal layerthat is substantially thicker than any of the other metal layers. Thethick metal layer for example may be at least 3 times thicker than thenext thickest metal layer of the metal stack, advantageously at least 4times thicker, more advantageously at least 5 times thicker, and may be6-8 times thicker, or more. This thick metal layer may be employed, forexample, as a power plane in CMOS circuitry and in embodiments hereinthe thick metal layer is further utilized in the MEMS structure toadvantageously increase the mass of a released proof mass.

FIG. 1A is an isometric view of a thin metal stack 107 including one ormore thin metal layers in regions of MEMS 100, in accordance with anembodiment. Both proof mass 105 and frame 110 are of a same thin metalstack 107. Where stack 107 includes more than one thin metal layer, eachmetal layer of the stack has substantially the same patterned featuresin vertical alignment with underlying layer(s) and overlying layers(s).The proof mass 105 includes a body region 106 and a sensing structure115 connected to body region 106. Within body region 106, lower levelopenings 125 pass through at least one metal layer in thin metal stack107. In the exemplary embodiment, openings 125 pass through all metallayers in thin metal stack 107 to expose an underlying substrate 103.Lower level openings 125 are arrayed over body region 106 and may beutilized for release of body region 106 from substrate 103 (i.e.,“release holes”). Openings 125 have a first lateral critical dimensionCD₀, which may vary with design rules of a process technology node.Openings 125 are further spaced apart by a lateral distance S₀ that maysimilarly satisfy one or more design rules (e.g., pattern density),and/or ensure proper release of proof mass 105 from substrate 103.

Sensing structure 115 includes a plurality of comb fingers extendingoutward from body region 106 by length L₁ along the width W. The combfingers, also being of the metal stack 107 are patterned with finegeometries, advantageously with the smallest geometries permitted bydesign rules of the process technology node for each successive thinmetal interconnect layer making up stack 107. For example, an individualcomb finger may have a width CD less than 1 μm, while L₁ is tens ofmicrons. A gap G separates sensing structure 115 from a second sensingstructure 116 that is connected to frame 110 disposed over a second areaof substrate 103. Gap G may be the smallest spacing (e.g.,advantageously less than 0.5 μm) permitted by design rules of theprocess technology node for each successive thin metal interconnectlayer making up stack 107. Sensing structure 116 includes anotherplurality of comb fingers extending outward from frame 110 by length L₁along the width W. The comb fingers, also being of the metal stack 107,are patterned with fine geometries to interdigitate with those ofsensing structure 115 forming interdigitated comb 117. A change incapacitance between sensing structures 115, 116 may be utilized to sensedisplacement of proof mass 105 relative to frame 110.

As further illustrated in FIG. 1A, proof mass 105 is physically coupledto frame 110 through spring 120 spanning a width W₁ between proof mass105 and frame 110. Spring 120 is patterned in one or more thin metallayers (and intervening dielectrics) of metal film stack 107. Thespecific structure of spring 120 is highly dependent on the desiredmechanical and electrical coupling/isolation between proof mass 105 andframe 110 and embodiments herein are not limited to any specific springarchitecture.

Frame 110 further includes a body region 111A, adjacent to a first sideof the proof mass 105 of width W, joined to a second body region 111Bthat adjacent to a second side of the proof mass 105 of length L. Secondlower level openings 126 pass through at least one metal layer withinthe body regions 111A, 111B. Each opening 126 has a second lateraldimension CD₁ and is spaced apart from a nearest neighboring opening bya lateral distance S₁. In the exemplary embodiment where frame 110 is(partially) released from substrate 103, openings 126 may also functionas release holes, however both CD₁ and S₁ are significantly greater thanCD₀ and S₀, respectively. Larger dimensioned openings 126 advantageouslyreduce the mass of frame 110, and, along with spacing S₁, may furthertune deflection of frame 110 relative to proof mass 105.

In exemplary embodiments, a frame further includes extension structures.In FIG. 1, frame 110 includes a comb extension structure 128 and aspring extension structure 129. Comb extension structure 128 includesfingers passing through the lateral width of frame body region 111B thatare of a similar lateral dimensions as that of interdigitated combs 117(e.g. L₂=L₁, finger width, and gap spacing is substantially the same).Fingers in comb extension structure are not cantilevered beams, butinstead are anchored at opposite ends to frame body regions. Combextension structure 128 is further in lateral alignment (e.g., iny-dimension) with interdigitated combs 117. Likewise, spring extensionstructure 129 includes fingers passing through the lateral width offrame body region 111A that has similar lateral dimensions to spring 120(e.g., W₂=W₂). Spring extension structure 129 is further in lateralalignment (e.g., in x-dimension) with spring 120. Noting extensionstructures 128, 129 lack a direct coupling function akin to that ofinterdigitated combs 117 or spring 120, incorporation of thesestructures is advantageous for matching film stress in frame 110 withthat in proof mass 105. This is particularly advantageous where frame110 is released front substrate 103 in a manner that renders frame body111B susceptible to static deflection to or from substrate 103, asdescribed further elsewhere herein. Extension structures 128, 129increase structural similarity between frame 110 and proof mass 105 thathelps to match static deflection of frame 110 to that of proof mass 105.

In embodiments, a thick metal layer is disposed over a thin metal layerin only certain portions of the MEMS. FIG. 1B is an isometric view of athick metal layer 150 disposed over thin metal stack 107 in certainregions of the MEMS 100. In general, metal stack 107 is covered withthick metal layer 150 within proof mass body region 106 as well as framebody regions 111A, 111B. Thick metal layer 150 may be at least 3 timesthicker than any thin metal layer of metal stack 107, advantageous atleast 4 times thicker, more advantageously at least 5 times thicker, andmay be 6-8 times thicker, or more. Thick metal layer 150 is patternedsuch that certain regions of MEMS 100 have only thin metal layers. Asshown in FIG. 1B, thick metal 150 is absent from sensing structures 115,116, and is also absent from spring 120. Because of greater thickness,thick metal 150 may be patterned with insufficient fidelity to achieveadvantageously high surface area and small gap spacing between sensingstructures 115, 116. Similarly, thick metal pattern fidelity may beinsufficient to achieve a desired mechanical coupling in spring 120.Nevertheless, body region 106 includes thick metal 150, which mayincrease mass significantly above that of metal stack 107 alone.

Within body region 106, openings 135 pass through thick metal layer 150.In the exemplary embodiment, large openings 135 are distributed (e.g.,in a two dimensional array) over proof mass body region 106. Each largeopening 135 has a critical lateral dimension CD₂ and adjacent largeopenings 135 are spaced apart by a lateral distance S₂. In embodiments,CD₂, is significantly larger than gap G. In further embodiments, CD₂ isalso significantly larger than the lateral dimension CD₀ andadvantageously larger than CD₀+S₀ such that more than one lower levelopening 125 is disposed within a single opening 135, as depicted in FIG.1B. In certain such embodiments, S₂ is also significantly larger thangap G, and larger than distance S₁. Lateral dimension CD₂ and spacing S₂may, for example, be set to minimum feature pitch and/or pattern densitydesign rules for thick metal layer 150.

Large openings 136 are similarly distributed (e.g., linearly arrayed)over frame body regions 111A, 111B. Each large opening 136 has acritical lateral dimension CD₃ and adjacent large openings 136 arespaced apart by a lateral distance S₃. In embodiments, CD₃ isapproximately equal CD₂ as guided by the same design rule. Similarly, inthe exemplary embodiments S₃ is equal to S₂. With stress in frame 110being a strong function of film stress in thick metal 150, dimensioningand spacing openings 136 in a manner that is similar to openings 135advantageously matches deflection in frame 110 to that of proof mass105. In certain such embodiments, the lateral spacing S₁ of lower levelopenings 126 is approximately equal to S₃ and lateral dimension CD₁ ofopening 126 is at least approximately equal to CD₃ such that opening 126is approximately the same size as opening 136, ensuring a largereduction in mass of frame 110.

In embodiments, a frame includes thin metal regions disposed in lateralalignment with thin metal regions present in structures that couple thefame to the proof mass (e.g., electrical coupling through a sensingstructure and mechanical coupling through a spring structure). Infurther embodiments, a thin metal flame region has a same lateraldimension as a thin metal region of the coupling structure to which itis aligned. In the exemplary embodiment illustrated in FIG. 1B, thinmetal frame region 137 is disposed along frame body region 111A inlateral alignment with interdigitated comb 117 such that thick metalsidewall 150B is coincident with comb extension 128. Because thick metallayer 150 is absent over comb extension structure 128, thin metal flameregion 137 has approximately the same lateral dimension (e.g., iny-dimension) as extension structure 128 and interdigitated combs 117(i.e., L₁=L₂=L₃). Similarly, thin metal frame region 140 is laterallyaligned (e.g. in x-dimension) with spring 120 and has approximately thesame lateral dimension (e.g., in x-dimension) as spring extensionstructure 129 and the span of spring 120 (i.e., W₁=W₂=W₃). As with theextension regions, the thin metal frame regions are advantageous formatching film stress in frame 110 with that in proof mass 105. Thinmetal frame regions 137, 140 are independent of extension structures128, 129 such that a thin metal frame region need not also be patternedinto an extension structure, and a patterned extension structure neednot be flee of the thick metal layer. The exemplary thin metal extensionregion however is particularly advantageous where frame 110 is releasedfrom substrate 103 in a manner that permits static deflection of flamebody 111B to or from substrate 103 because static deflection of proofmass 105 may be well-matched to that of a frame 110 having thin metalframe regions 137, 140 patterned into extension structures 128, 129.

FIG. 2 is an isometric view of MEMS 200, in accordance with oneaccelerometer embodiment employing the MEMS structures described in thecontext of FIG. 1B. As shown, frame 110 entirely surrounds proof mass105. Thick metal layer 150 is patterned with large openings 136 in framebody regions 111A, 111B, 111C, 111D to match lateral dimensions andlateral spacing of openings 135 in proof mass 105. Thin metal frameregions 140A and 140B stripe opposite ends of interdigitated combs 117A,117B on first opposing sides of proof mass 105. Similarly, thin metalframe regions 137A and 137B snipe opposite ends of springs 120A. 120B ona second opposing sides proof mass 105. Two opposing frame body regions111B, 111D are anchored to substrate 103 at anchor points 230A, 230Bwith the remainder of frame 110 released from substrate 103 to suspendfrom the anchor points 230A, 230B along with poof mass 105. The massdifferential between proof mass 105 and frame 110 therefore stems fromthe differential in substrate area occupied by proof mass 105 and frame110, as well as the differential in size of lower level openings 125 and126.

In embodiments where both a proof mass and a frame are released, theframe and proof mass have approximately the same static deflectionrelative to an underlying substrate. FIG. 3A is an isometric view of areleased MEMS proof mass and released frame having approximately equalstatic deflection from a substrate 103, in accordance with anembodiment. The dashed line represents position of the proof mass 305and frame 310 at a time prior to release from substrate 103 and thesolid line drawing depicts position of proof mass 305 and frame 310 at atime following release from substrate 103. As shown, the free ends ofproof mass 305 and frame 310 statically deflect from a plane ofsubstrate 103 by an amount Δz that is not uniform (e.g., varying from0.1 μm-1 μm). For best sensing by interdigitated combs 317, proof mass305 and frame 310 should have nearly identical curvature along anycantilevered lateral lengths to be follow substantially a same surfaceS. For such embodiments, the extension structures and thin metal regionspreviously described advantageously permit stress matching between theframe and proof mass to achieve the nearly identical static deflectioncurvature that properly positions the sensing structures.

In embodiments, a MEMS gyroscope or accelerometer includes a releasedproof mass electrically coupled to CMOS circuitry formed on the samesubstrate as the MEMS gyroscope or accelerometer. In the exemplaryembodiment, the MEMS gyroscope or accelerometer includes a releasedproof mass and/or frame comprising the same thin film metal interconnectlayers present in the CMOS circuitry. FIG. 3B is a cross-sectional viewof a MEMS proof mass employing a thick metal layer, in accordance withan embodiment. FIG. 3B further illustrates one embodiment applicable tothe MEMS structures depicted in FIGS. 1A-1B. In the exemplaryembodiment, thick metal 150 is a top, capping layer of metal film stack180. Thick metal layer 150 is disposed over a plurality of thin metallayers 307A-307N that make up thin metal stack 107. Dielectric layers309A-309N separate each thin metal layer 307A-307N, and further separatethick metal layer 150 from a topmost thin metal layer 309N. FIG. 3Billustrates both a portion of proof mass body region 106 and a CMOSregion 302. Within proof mass body legion 106, large opening 135 exposesa plurality of lower level openings 125. Each opening 125 passes througheach thin metal layer 307A-307N, as well as through interveningdielectric layers 309A-309N, to expose substrate 103. In embodiments, atleast one of the substrate or a dielectric layer between a thin metallayer and the substrate is removed to release the proof mass. In theexemplary embodiment depicted in FIG. 3B, thin metal stack 107 isreleased as a result of recess 340 having been etched into a portion ofsubstrate 103. CMOS region 302 is adjacent to a MEMS employing proofmass body region 106, CMOS circuitry including a plurality of MOStransistors 360 is interconnected with layers of thin metal stack 107disposed within CMOS area 302. CMOS area 302 may further include thickmetal layer 105, for example as a power plane for the CMOS circuitry.

FIG. 3C is a cross-sectional view of a frame body region 111A employingthick metal layer 150, in accordance with an embodiment. FIG. 3Cillustrates one exemplary embodiment where thick metal layer 150 is acapping layer over thin metal film stack 107, just as for proof massbody 106 depicted FIG. 3B. Large opening 136 is vertically aligned withlower level opening 126. Lower level opening 126 extends through allthin metal layers of thin metal stack 107, and exposes substrate 103. Inadvantageous embodiments CD₃ is substantially equal to CD₂.

FIG. 4 is a flow diagram illustrating a method 401 of fabricating a MEMSsensor with a thick interconnect metal layer, in accordance with anembodiment. Method 401 begins with receiving a substrate at operation405. The substrate may be any semiconductor (e.g., silicon),semiconductor-on-insulator (SOI), other conventional material. In oneexemplary embodiment, the substrate is silicon and includes MOStransistors. At operation 410, body regions of a proof mass and frameare patterned into a metal film stack that includes a thick metal layerseparated from a thin metal layer by a dielectric. In embodiments, thepatterning operation 410 is performed concurrently with patterning ofthe metal film stack into interconnects for the MOS transistors to formCMOS circuitry. Patterning of the metal layers may proceed byconventional damascene processing as is typically performed in CMOStechnology. For example, openings are formed through a dielectric layer,and a fill metal, such as copper, plated up in the dielectric openings.Where the metal film stack includes a plurality of thin metal layers,patterning operation 410 may be an iterative process with patterning ofessentially the same feature in each of a plurality of thin metallayers. Features formed in the metal layers may be vertically alignedacross successive metal layers, for example as is depicted in FIG. 5Aillustrating a structure in a proof mass body region following operation410. As shown, two thin metal layers are patterned to have verticallyaligned features 506A, 507A laterally separated from vertically alignedfeatures 506B, 507B with dielectric 508 disposed there between.Patterning of the thick metal layer proceeds in a similar manner uponcompleting the thin metal film stack. In FIG. 5A, thick metal layer 550is patterned to have dielectric 509 vertically aligned with dielectric508. At operation 420, coupling structures (e.g., sensing structures andsprings, etc.) are patterned into a metal film stack that includes athin metal layer, but lacks the thick metal layer. In embodiments,patterning operation 420 is performed concurrently with patterningoperation 410 with dielectric layer patterning and metal fill proceedingas for the body regions except no top level openings are formed toreceive a thick metal fill over the coupling structures and extensionstructures. At operation 430, exposed dielectric is etched selectivelyto the metal layers to form openings in the thick metal verticallyaligned with openings in the underlying thin metal layers. Openings inthe underlying layers are further formed by continuing the etch processto remove any dielectric exposed between vertically aligned thin metalfeatures. These openings may serve as release holes, or solely forweight relief and/or tuning of a features static deflection. Inembodiments, the openings in the thick metal layer have a lateral CD atleast three times larger than a lateral CD of an underlying openingpatterned in the thin metal layers. FIG. 5B is a cross-sectional view ofthe structure that was depicted in FIG. 5A subsequent to operation 430,in accordance with an embodiment. As shown, removal of dielectric 509results in opening 135 that exposes thin metal features 507A, 507B. Theetch at operation 430 further removes dielectric 508 selectively tometal features 507A, 507B to form lower level opening 125 that passesvertically through the plurality of thin metal layers. Because the metallayers are separated by dielectric, the etch operation 430 isadvantageously anisotropic so as to avoid undercutting individual metallayers during formation of opening 125. In the exemplary embodiment, theetch is a plasma etch process capable of high aspect ratio (HAR)etching. In regions where no thick metal is present, for example overcoupling structures, dielectric etching at operation 430 also opens thegap G between comb fingers and lengths of coupling springs. Method 401completes at operation 440 with any additional etching needed to fullyrelease the proof mass and frame from the substrate. For example, anisotropic wet chemical, vapor phase, or plasma etch may be performed toselectively remove a sacrificial material, such as substratesemiconductor, to form the substrate void depicted in FIGS. 2B, 2C.

FIGS. 6A-6C are views of a MEMS gyroscope 601 employing a thick metallayer, in accordance with another embodiment. The gyroscope 601 has anumber of additional design attributes relative to MEMS 200 describedelsewhere herein to emphasize the breadth of the architectures andtechniques recited in the claims. FIG. 6A is a partial plan view offeatures formed in a thin metal layer (or stack of multiple suchlayers). As shown, gyroscope 601 again includes a plurality oflower-level openings 125 arrayed over a body region proof mass 105.Proof mass 105 is mechanically coupled to frame 110 through spring 120.Proof mass 105 is electrically coupled to frame 110 through a sensingstructure comprising interdigitated combs 117, as previously described.Frame 110 has openings 126 spaced apart a distance S₂. Frame 110 ismechanically coupled to an outer frame 610 through spring 620. Outerframe 610 includes openings 626 having lateral dimensions and spacingequal to that of openings 126. Outer frame 610 is further mechanicallycoupled to substrate 103 by anchor point 230 and electrically coupled toframe 110 by a second sensing structure comprising interdigitated combs617. Both flames 110 and 610 include patterned comb extension structures128 and spring extension structures 129, 629, which may improve matchingof film stresses in frames 110 and 610 and in proof mass 105. This isparticularly advantageous in the exemplary embodiment where frames 110and 610 are released from substrate 103. As further shown in FIG. 6A,spring 620 and spring extension structure 129 are confined to a lateralCD approximately equal to that of interdigitated comb 617, for exampleto further match film stresses in frames 110 and 610 and proof mass 105.

FIGS. 6B and 6C are isometric views of the MEMS device 601 furtherillustrating thick metal layer 150 disposed over regions of the thinmetal layer. As shown, proof mass body region includes a plurality oflarge openings 135 in thick metal layer 150. Each large opening exposesfour of the lower level openings 125. Frames 110 and 610 further includea plurality of large openings 136 and 636, respectively. Openings 636have a lateral dimension that is substantially equal to that of opening136 (e.g., CD₃=CD₄). Openings 136 and 636 are vertically aligned withopenings 126, 626, respectively. Both frames 110 and 610 further includethin metal frame regions 137, 637, 140, and 640, for example to matchfilm stresses in frames 110 and 610 and proof mass 105.

FIG. 7 illustrates an isometric view of a computing device platform 700and schematic view 721 of a microelectronic device 710 employed by theplatform, in accordance with an embodiment of the present invention. Thecomputing platform 700 may be any portable device configured for each ofelectronic data display, electronic data processing and wirelesselectronic data transmission. For exemplary mobile embodiments, thecomputing platform 700 may be any of a tablet, a smart phone, laptop orultrabook computer, etc., further including a display screen 705 thatmay be a touchscreen (capacitive, inductive, resistive, etc.), achip-level (SoC) or package-level integrated microelectronic device 710,and a battery 713. The integrated device 710 is further illustrated inthe expanded view 721. In the exemplary embodiment, the device 710includes at least one memory chip and at least one processor chip (e.g.,a multi-core microprocessor and/or graphics processor cores 730, 731).In embodiments, an integrated MEMS accelerometer or gyroscope 732employing a thick metal layer, for example as described in more detailelsewhere herein (e.g., FIGS. 1A, 1B, etc.), is integrated into thedevice 710. The device 710 is further coupled to the board, substrate,or interposer 500 along with, one or more of a power managementintegrated circuit (PMIC) 715, RF (wireless) integrated circuit (RFIC)725 including a wideband RF (wireless) transmitter and/or receiver(e.g., including a digital baseband and an analog front end modulefurther comprising a power amplifier on a transmit path and a low noiseamplifier on a receive path), and a controller thereof 711.Functionally, the PMIC 715 performs battery power regulation, DC-to-DCconversion, etc., and so has an input coupled to the battery 713 andwith an output providing a current supply to all the other functionalmodules, including the MEMS 732.

As further illustrated, in the exemplary embodiment the RFIC 725 has anoutput coupled to an antenna to provide to implement any of a number ofwireless standards or protocols, including but not limited to Wi-Fi(IEEE 802.11 family), WiMAX (IEEE 802.16 family), IEEE 802.20, long termevolution (LTE), Ev-DO, HSPA+, HSDPA+, HSUPA+, EDGE, GSM, GPRS, CDMA,TDMA, DECT, Bluetooth, derivatives thereof, as well as any otherwireless protocols that are designated as 3G, 4G, 5G, and beyond. Inimplementations, each of these modules, including the MEMS structure andCMOS circuitry, may be integrated onto a single chip as an SoC, ontoseparate ICs coupled to a package substrate of the packaged device 710,or at a board level.

FIG. 8 is a functional block diagram of a computing device 1000 inaccordance with one embodiment of the invention. The computing device1000 may be found inside the platform 700, for example, and furtherincludes a board 1002 hosting a number of components, such as but notlimited to a processor 1004 (e.g., an applications processor) and atleast one communication chip 1006. In embodiments, at least theprocessor 1004 is integrated (e.g., in-package) with a MEMSaccelerometer and/or gyroscope in accordance with embodiments describedelsewhere herein. The processor 1004 is physically and electricallycoupled to the board 1002. The processor 1004 includes an integratedcircuit die packaged within the processor where the tem “processor” mayrefer to any device or portion of a device that processes electronicdata from registers and/or memory to transform that electronic data intoother electronic data that may be stored in registers and/or memory.

In some implementations the at least one communication chip 1006 is alsophysically and electrically coupled to the board 1002. In furtherimplementations, the communication chip 1006 is part of the processor1004. Depending on its applications, computing device 1000 may includeother components that may or may not be physically and electricallycoupled to the board 1002. These other components include, but are notlimited to, volatile memory (e.g., DRAM), non-volatile memory (e.g., RAMor ROM) in the form of flash memory or STTM, etc., a graphics processor,a digital signal processor, a crypto processor, a chipset, an antenna,touchscreen display, touchscreen controller, battery, audio codec, videocodec, power amplifier, global positioning system (GPS) device,integrated inertial sensor, accelerometer, speaker, camera, and massstorage device (such as hard disk drive, solid state drive (SSD),compact disk (CD), digital versatile disk (DVD), and so forth). At leastone of the communication chips 1006 enables wireless communications forthe transfer of data to and from the computing device 1000. The term“wireless” and its derivatives may be used to describe circuits,devices, systems, methods, techniques, communications channels, etc.,that may communicate data through the use of modulated electromagneticradiation through a non-solid medium. The term does not imply that theassociated devices do not contain any wires, although in someembodiments they might not. The commination chip 1006 may implement anyof a number of wireless standards or protocols, including but notlimited to those described elsewhere herein. The computing device 1000may include a plurality of communication chips 1006. For instance, afirst communication chip 1006 may be dedicated to shorter-range wirelesscommunications such as Wi-Fi and Bluetooth and a second communicationchip 1006 may be dedicated to longer-range wireless communications suchas GPS, EDGE, GPRS, CDMA, WiMAX, LTE, Ev-DO, and others.

It will be recognized that the invention is not limited to theembodiments so described, but can be practiced with modification andalteration without departing from the scope of the appended claims. Forexample, the above embodiments may include specific combination offeatures.

In one exemplary embodiment, a Microelectromechanical System (MEMS),comprises a substrate, a released proof mass disposed over a first areaof the substrate and having a first body region. The first body regionmay further comprise a thick metal layer over a thin metal layer, thethick metal layer having a film thickness that is at least five timesgreater than that of the thin metal layer, and a first sensing structurecomprising the thin metal layer, but lacking the thick metal layer. TheMEMS may further include a frame disposed over a second area of thesubstrate, the frame having, a second body region comprising at leastone of the metal layers, and a second sensing structure comprising thethin metal layer, but also lacking the thick metal layer, and separatedfrom the first sensing structure by a gap to register a change incapacitance with displacement of the proof mass relative to the frame.

In a further embodiment, the second body region comprises the thin metallayer and the thick metal layer. The frame is at least partiallyreleased from the substrate, and the first and second body regions areboth statically deflected from a surface of the substrate.

In a further embodiment, the second body region comprises the thin metallayer and thick metal layer. The first body region further comprises afirst plurality of large openings distributed over the first bodyregion, each of the first large openings extending through the thickmetal layer, having a first lateral dimension that is larger than thegap, and spaced apart by a first spacing. The second body region furthercomprises a second plurality of the large openings distributed over thesecond area of the substrate, each of the second plurality of largeopenings extending through the thick metal layer, and spaced apart bythe first spacing. In a further embodiment, the second body regioncomprises the metal film stack, including the thick metal layer. Thefirst body region further comprises a first plurality of large openingsdistributed along a first dimension of the substrate, each of the firstlarge openings extending through the thick metal layer, having a firstlateral dimension that is larger than the gap, and spaced apart by afirst spacing. The second body region further comprises a secondplurality of the large openings distributed along the first dimension,each of the second large openings extending through the thick metallayer, and spaced apart by the first spacing. At least one of the firstlarge openings exposes a first lower level opening that extends throughat least the thin metal layer and exposes the substrate. The first lowerlevel opening has a second lateral dimension smaller than the firstlateral dimension. At least one of the second large openings exposes asecond lower level opening that extends through at least the thin metallayer and exposes the substrate. The first lower level opening has athird lateral dimension larger than the second lateral dimension.

In a further embodiment, the second body region comprises the metal filmstack, including the thick metal layer. The first body region furthercomprises a first plurality of large openings distributed over the firstsubstrate area, each of the first large openings extending through thethick metal layer, having a first lateral dimension, and spaced apart bya first spacing that is larger than the gap. At least one of the firstlarge openings exposes a plurality of first lower level opening thatextend through a plurality of underlying thin metal layers in the stackand expose the substrate. The second body region further comprises asecond plurality of the large openings distributed across the secondsubstrate area, each of the second large openings extending through thethick metal layer and spaced apart by the first spacing. One of thesecond large openings exposes only a single second lower level openingthat extends through a plurality of underlying thin metal layers in thestack and exposes the substrate.

In a further embodiment, the frame has a first frame side adjacent to alength of the proof mass, and has a second frame side adjacent to awidth of the proof mass, and orthogonal to the first frame side. Thefirst sensing structure comprises a first plurality of comb fingersextending from the first body region toward the first frame side anddistributed along the length. The second sensing structure comprises asecond plurality of comb fingers along the first side, extending fromthe second body region toward the proof mass, and interdigitated withthe first plurality of fingers. The frame comprises thick metal frameregions including the thick metal layer, and thin metal flame regionslacking the thick metal layer. A first of the thin metal frame regionsis disposed along the second frame side in alignment with the first andsecond fingers and has a thickness and a lateral dimension approximatelyequal to that of the first and second comb fingers.

In a further embodiment, the frame has a first flame side adjacent to alength of the proof mass, and has a second frame side adjacent to awidth of the proof mass. The proof mass is physically coupled to theframe through a spring spanning a distance between the proof mass to thefirst flame side. The spring comprises at least the thin metal layer,but lacks the thick metal layer. The frame comprises thick metal frameregions including the thick metal layer, and thin metal frame regionslacking the thick metal layer A first of the thin metal frame regions isdisposed along the first frame side in alignment with the spring, andthe first thin metal frame region has a thickness and a lateraldimension approximately equal a span of the spring.

In a further embodiment, the frame has a first frame side adjacent to alength of the proof mass, and has a second frame side adjacent to awidth of the proof mass. The first sensing structure comprises a firstplurality of comb fingers extending from the first body region towardthe first frame side and distributed along the length. The secondsensing structure comprises a second plurality of comb fingers along thefirst side, extending from the second body region toward the proof mass,and interdigitated with the first plurality of fingers. The proof massis physically coupled to the frame through a spring spanning a distancebetween the proof mass to the first frame side. The spring comprises atleast the thin metal layer, but lacks the thick metal layer. The framecomprises thick metal frame regions including the thick metal layer andthin metal frame regions lacking the thick metal layer. A first of thethin metal frame regions is disposed along the second frame side inalignment with the first and second comb fingers and has a dimensionapproximately equal to that of the first and second comb fingers. Asecond of the thin metal frame regions is disposed along the first frameside in alignment with the spring. The second thin metal frame regionhas a thickness and lateral dimension approximately equal a span of thespring.

In embodiments, an integrated accelerometer, includes the MEMS describedin any of the exemplary embodiments above, and at least one of anamplifier or acceleration calculator electrically coupled to the MEMSand comprising integrated circuitry including transistors disposedwithin the substrate.

In embodiments, a Microelectromechanical System (MEMS) gyroscope oraccelerometer, comprises a substrate including CMOS circuitry, areleased proof mass, and a frame. The released proof mass iselectrically coupled to the CMOS circuitry and formed in thin film metalinterconnect layers over the CMOS circuitry, and includes a plurality offirst coupling fingers cantilevered from the substrate. The proof massincludes a first body region including a thick metal layer separatedfrom a thin metal layer. The thick metal layer has a film thickness thatis at least five times greater than that of the thin metal layer. Thefirst coupling fingers comprise the thin metal layer, but lack the thickmetal layer. The frame surrounds at least two sides of the proof massand electrically couples to the CMOS circuitry and forms the thin filmmetal interconnect layers. The frame includes a plurality of secondcoupling fingers interdigitated with the first coupling fingers toregister a change in capacitance in response to deflection in one ormore spring physically connecting the proof mass to the frame. The oneor more spring comprises at least the thin metal layer, but lacks thethick metal layer.

In further embodiments, the proof mass further comprises a firstplurality of large openings distributed over the first area of thesubstrate, each of the first large openings extending through the thickmetal layer, having a first lateral dimension, and spaced apart by afirst spacing that is larger than the gap between the first and secondcapacitive fingers. The frame further comprises a second plurality ofthe large openings distributed over a second area of the substrate, eachof the second plurality of large openings extending through the thickmetal layer, and spaced apart by the first spacing.

In further embodiments, the proof mass and frame have approximately thesame static deflection away from the substrate to maintain the firstcoupling fingers in adjacent to the second coupling fingers over alength of the proof mass.

In embodiments, a mobile computing platform comprises the MEMS gyroscopeor accelerometer in any of the exemplary embodiments above, a displayscreen to display output dependent on an acceleration registered byaccelerometer, and a wireless transceiver to relay information dependenton a condition registered by the gyroscope or accelerometer.

In embodiments, a method of forming a Microelectromechanical System(MEMS), the method comprises receiving a substrate, patterning bodyregions of a proof mass and surrounding frame, patterning sensingstructures and releasing the proof mass from the substrate. Body regionsof a proof mass and surrounding frame are patterned into a metal filmstack including a thick metal layer separated from a thin metal layer bydielectric, the thick metal layer having a film thickness that is atleast five times greater than that of the thin metal layer. The sensingstructures of the proof mass and frame are patterned into a metal filmstack including the thin metal layer, but lacking the thick metal layer.

In further embodiments, releasing the proof mass further comprisesreleasing the frame to allow the frame to deflect from the substrate byan amount approximately equal to the deflection experienced by the proofmass in response to static stress in the metal film stack.

In further embodiments, the method further comprises patterning a springcoupling between the proof mass and frame into a metal film stackincluding the thin metal layer, but lacking the thick metal layer.Releasing the proof mass further comprises releasing the frame to allowthe frame to deflect from the substrate by an amount approximately equalto the deflection experienced by the proof mass in response to staticstress in the metal film stack.

In further embodiments, the thick metal layer is a top level of metaldisposed over the thin metal layer. The thin metal layer is one of aplurality of thin metal layers within the metal film stack.

In further embodiments, the method includes anisotropically etchingexposed dielectric within the body region of the frame selectively tothe metal layers to form large openings having a first criticaldimension (CD) through the thick metal layer and through the thin metallayer. In these further embodiments, the method also includesanisotropically etching exposed dielectric within the body region of theproof mass to form the large openings through the thick metal layervertically aligned with a small opening through the thin metal layerhaving a second CD, smaller than the first CD.

In further embodiments, the method further includes anisotropicallyetching exposed dielectric within the body region of the proof massselectively to the metal layers to form a large opening having a firstcritical dimension (CD) through the thick metal layer vertically alignedwith a plurality of small openings through the thin metal layer having asecond CD, smaller than the first CD. In these further embodiments, themethod also includes anisotropically etching exposed dielectric withinthe body region of the frame selectively to the metal layers to form alarge opening having the first CD through the thick metal layervertically aligned with a single opening through the thin metal layer.

In further embodiments, patterning sensing structures of the proof massand frame further comprises forming first comb fingers connected to thebody region of the proof mass and second comb fingers connected to thebody region of the frame and interdigitated with the first comb fingers.In these further embodiments, the method further comprisesanisotropically etching exposed dielectric between the first and secondcomb fingers to form a gap there between.

However, the above embodiments are not limited in these regards and, invarious implementations, the above embodiments may include theundertaking only a subset of such features, undertaking a differentorder of such features, undertaking a different combination of suchfeatures, and/or undertaking additional features than those featuresexplicitly listed. The scope of the invention should, therefore, bedetermined with reference to the appended claims, along with the fullscope of equivalents to which such claim are entitled.

What is claimed is:
 1. A Microelectromechanical System (MEMS),comprising: a substrate; a released proof mass disposed over a firstarea of the substrate and having: a first body region comprising a thickmetal layer over a thin metal layer, the thick metal layer having a filmthickness that is at least five times greater than that of the thinmetal layer; and a first sensing structure comprising the thin metallayer, but lacking the thick metal layer; and a frame disposed over asecond area of the substrate, the frame having: a second body regioncomprising at least one of the metal layers; and a second sensingstructure comprising the thin metal layer, but also lacking the thickmetal layer, and separated from the first sensing structure by a gap toregister a change in capacitance with displacement of the released proofmass relative to the frame.
 2. The MEMS of claim 1, wherein: the secondbody region comprises the thin metal layer and the thick metal layer;the frame is at least partially released from the substrate; and thefirst and second body regions are both statically deflected from asurface of the substrate.
 3. The MEMS of claim 1, wherein: the secondbody region comprises the thin metal layer and thick metal layer; thefirst body region further comprises a first plurality of large openingsdistributed over the first body region, each of the first large openingsextending through the thick metal layer, having a first lateraldimension that is larger than the gap, and spaced apart by a firstspacing; and the second body region further comprises a second pluralityof the large openings distributed over the second area of the substrate,each of the second plurality of large openings extending through thethick metal layer, and spaced apart by the first spacing.
 4. The MEMS ofclaim 1, wherein: the second body region comprises a metal film stack,including the thick metal layer; the first body region further comprisesa first plurality of large openings distributed along a first dimensionof the substrate, each of the first large openings extending through thethick metal layer, having a first lateral dimension that is larger thanthe gap, and spaced apart by a first spacing; and the second body regionfurther comprises a second plurality of the large openings distributedalong the first dimension, each of the second large openings extendingthrough the thick metal layer, and spaced apart by the first spacing,wherein: at least one of the first large openings exposes a first lowerlevel opening that extends through at least the thin metal layer andexposes the substrate, the first lower level opening having a secondlateral dimension smaller than the first lateral dimension; and at leastone of the second large openings exposes a second lower level openingthat extends through at least the thin metal layer and exposes thesubstrate, the first lower level opening having a third lateraldimension larger than the second lateral dimension.
 5. The MEMS of claim1, wherein: the second body region comprises a metal film stack,including the thick metal layer; the first body region further comprisesa first plurality of large openings distributed over the first substratearea, each of the first large openings extending through the thick metallayer, having a first lateral dimension, and spaced apart by a firstspacing that is larger than the gap, wherein at least one of the firstlarge openings exposes a plurality of first lower level openings thatextend through a plurality of underlying thin metal layers in the stackand expose the substrate; and the second body region further comprises asecond plurality of the large openings distributed across the secondsubstrate area, each of the second large openings extending through thethick metal layer and spaced apart by the first spacing, wherein one ofthe second large openings exposes only a single second lower levelopening that extends through a plurality of underlying thin metal layersin the stack and exposes the substrate.
 6. The MEMS of claim 1, wherein:the frame has a first frame side adjacent to a length of the proof mass,and has a second frame side adjacent to a width of the proof mass, andorthogonal to the first frame side; the first sensing structurecomprises a first plurality of comb fingers extending from the firstbody region toward the first frame side and distributed along thelength; the second sensing structure comprises a second plurality ofcomb fingers along the first side, extending from the second body regiontoward the proof mass, and interdigitated with the first plurality offingers; and the frame comprises: thick metal frame regions includingthe thick metal layer; and thin metal frame regions lacking the thickmetal layer, wherein a first of the thin metal frame regions is disposedalong the second frame side in alignment with the first and secondfingers and has a thickness and a lateral dimension approximately equalto that of the first and second comb fingers.
 7. The MEMS of claim 1,wherein: the frame has a first frame side adjacent to a length of theproof mass, and has a second frame side adjacent to a width of the proofmass; the proof mass is physically coupled to the frame through a springspanning a distance between the proof mass to the first frame side, thespring comprising at least the thin metal layer, but lacking the thickmetal layer; and the frame comprises: thick metal frame regionsincluding the thick metal layer; and thin metal frame regions lackingthe thick metal layer, wherein a first of the thin metal frame regionsis disposed along the first frame side in alignment with the spring, andwherein the first thin metal frame region has a thickness and a lateraldimension approximately equal a span of the spring.
 8. The MEMS of claim1, wherein: the frame has a first frame side adjacent to a length of theproof mass, and has a second frame side adjacent to a width of the proofmass; the first sensing structure comprises a first plurality of combfingers extending from the first body region toward the first frame sideand distributed along the length; the second sensing structure comprisesa second plurality of comb fingers along the first side, extending fromthe second body region toward the proof mass, and interdigitated withthe first plurality of fingers; the proof mass is physically coupled tothe frame through a spring spanning a distance between the proof mass tothe first frame side, the spring comprising at least the thin metallayer, but lacking the thick metal layer; and the frame comprises: thickmetal frame regions including the thick metal layer; and thin metalframe regions lacking the thick metal layer, wherein: a first of thethin metal frame regions is disposed along the second frame side inalignment with the first and second comb fingers and has a dimensionapproximately equal to that of the first and second comb fingers; and asecond of the thin metal frame regions is disposed along the first frameside in alignment with the spring, and wherein the second thin metalframe region has a thickness and lateral dimension approximately equal aspan of the spring.
 9. An integrated accelerometer, comprising: the MEMSof claim 1; and at least one of an amplifier or acceleration calculatorelectrically coupled to the MEMS and comprising integrated circuitryincluding transistors disposed on the substrate.
 10. AMicroelectromechanical System (MEMS) gyroscope or accelerometer,comprising: a substrate including CMOS circuitry; a released proof masselectrically coupled to the CMOS circuitry and formed in thin film metalinterconnect layers over the CMOS circuitry, and including a pluralityof first coupling fingers cantilevered from the substrate, wherein thereleased proof mass includes a first body region including a thick metallayer separated from a thin metal layer, the thick metal layer having afilm thickness that is at least five times greater than that of the thinmetal layer, and wherein the first coupling fingers comprise the thinmetal layer, but lack the thick metal layer; and a frame surrounding atleast two sides of the proof mass and electrically coupled to the CMOScircuitry and formed in the thin film metal interconnect layers, whereinthe frame includes a plurality of second coupling fingers interdigitatedwith the first coupling fingers to register a change in capacitance inresponse to deflection in one or more spring physically connecting theproof mass to the frame, the one or more spring comprising at least thethin metal layer, but lacking the thick metal layer.
 11. The MEMSgyroscope or accelerometer of claim 10, wherein: the proof mass furthercomprises a first plurality of large openings distributed over the firstarea of the substrate, each of the first large openings extendingthrough the thick metal layer, having a first lateral dimension, andspaced apart by a first spacing that is larger than the gap between thefirst and second coupling fingers; and the frame further comprises asecond plurality of the large openings distributed over a second area ofthe substrate, each of the second plurality of large openings extendingthrough the thick metal layer, and spaced apart by the first spacing.12. The MEMS gyroscope or accelerometer of claim 10, wherein the proofmass and frame have approximately the same static deflection away fromthe substrate to maintain the first coupling fingers adjacent to thesecond coupling fingers over a length of the proof mass.
 13. A mobilecomputing platform comprising: the MEMS gyroscope or accelerometer ofclaim 10; a display screen to display output dependent on anacceleration registered by accelerometer; and a wireless transceiver torelay information dependent on a condition registered by the gyroscopeor accelerometer.
 14. A method of forming a MicroelectromechanicalSystem (MEMS), the method comprising: receiving a substrate; patterningbody regions of a proof mass and a surrounding frame in a metal filmstack including a thick metal layer separated from a thin metal layer bydielectric, the thick metal layer having a film thickness that is atleast five times greater than that of the thin metal layer; andpatterning sensing structures of the proof mass and frame into a metalfilm stack including the thin metal layer, but lacking the thick metallayer; and releasing the proof mass from the substrate.
 15. The methodof claim 14, wherein releasing the proof mass further comprisesreleasing the frame to allow the frame to deflect from the substrate byan amount approximately equal to the deflection experienced by the proofmass in response to static stress in the metal film stack.
 16. Themethod of claim 14, wherein the method further comprises patterning aspring coupling between the proof mass and frame into a metal film stackincluding the thin metal layer, but lacking the thick metal layer; andwherein releasing the proof mass further comprises releasing the frameto allow the frame to deflect from the substrate by an amountapproximately equal to the deflection experienced by the proof mass inresponse to static stress in the metal film stack.
 17. The method ofclaim 14, wherein the thick metal layer is a top level of metal disposedover the thin metal layer, and wherein the thin metal layer is one of aplurality of thin metal layers within the metal film stack.
 18. Themethod of claim 14, further comprising: anisotropically etching exposeddielectric within the body region of the frame selectively to the metallayers to form large openings having a first critical dimension (CD)through the thick metal layer and through the thin metal layer; andanisotropically etching exposed dielectric within the body region of theproof mass to form the large openings through the thick metal layervertically aligned with a small opening through the thin metal layerhaving a second CD, smaller than the first CD.
 19. The method of claim14, further comprising anisotropically etching exposed dielectric withinthe body region of the proof mass selectively to the metal layers toform a large opening having a first critical dimension (CD) through thethick metal layer vertically aligned with a plurality of small openingsthrough the thin metal layer having a second CD, smaller than the firstCD; anisotropically etching exposed dielectric within the body region ofthe frame selectively to the metal layers to form a large opening havingthe first CD through the thick metal layer vertically aligned with asingle opening through the thin metal layer.
 20. The method of claim 14,wherein patterning sensing structures of the proof mass and framefurther comprises forming first comb fingers connected to the bodyregion of the proof mass and second comb fingers connected to the bodyregion of the frame and interdigitated with the first comb fingers; andwherein the method further comprises anisotropically etching exposeddielectric between the first and second comb fingers to form a gap therebetween.